I don't know if they really look alike or if it was the haircuts - or the type of characters they usually tend to play (hot, somewhat likeable princesses) - but I thought it was very clever to nab Christina Applegate and Reese Whitherspoon to play Jennifer Aniston's sisters in 'Friends.'

That last one's pretty funny. Perhaps more in the spirit OP intended, how about Elizabeth Montgomery and Erin Murphy on "Bewitched"? They even have the same initials!

Phil and Grant Mitchell on Eastenders.

DJ really looked like he could have been Roseanne and John Goodman's son. The other two looked totally adopted.
From different hemispheres.
And forget about Roseanne's lateral or ascending family. Again, different continents.

Mike Brady and his 3 sons.

The parents/parent equivalents and children in The Donna Reed Show, Father Knows Best, and Family Affair, all looked like they could be related to each other.

i thought jacki weaver and bradley cooper had a similar quality in silver linings playbook. why havent we ever seen helen hunt together with leelee sobieski or uma thurman with sarah polley?

Mary-LouiseParker and the kid who played her one armed son in Fried Green Tomatoes.

Jennifer Garner & Victor Garber

Jensen and Jared, Dean and Sam Winchester.

Robert Downey Jr and Winona Ryder as siblings in 1969

mary mcdonnell as mother of gyllenhaals in 'donnie darko'

Mary-Kate and Ashley Olsen.

Lauren Graham and Alexis Bledel

Charlie Sheen & Emilio Estevez

Barbara O'Neil (Ellen O'Hara) and Ann Rutherford (Carreen) in GONE WITH THE WIND

fairuza balk and ione skye in gas, food, lodging

Teri Garr and Lisa Kudrow

Lauren Graham and a pig would look related but not any of the actresses who have played her daughter

sondra locke and surrogate sister/lover ratboy in ratboy

None of the Cosby kids.

Kim Richards & Ike Eisenmann in those Witch Mountain movies.

precious and that bucket of fried chicken in precious

I always thought young Heather Tom looked like the daughter of Melody Thomas Scott and Eric Braeden.

The Chase family in My So Called Life. Especially the kid who wasn't Claire Danes.

I don't think it's necessary to do a spoiler alert on something that has already aired. DVR/Tivo people be damned

On Golden Pond

Oh%2C%20yeah...

Jennifer Connelly and Lily Collins as mother/daughter in their new indie movie. Forget its title.

The casts of The Cosby Show and Sister, Sister

What's the movie were Jena Malone played Mary Louise Parker's daughter? Was that "Saved"?

Same%20mannerisms%20and%20gestures.

Not a relative but the same person side bar:
I was always amazed at how spot on the actress who played young Dorothy in Golden Girls always was with her posture gestures, voice tone, inflections, and mannerisms.

R67 - Yes. Good casting choice.

[R68} the actress who played young Dorothy was/is Lyn Greene who later became a successful television producer

MLP and Jena Malone did not have the same mannerisms at all in Saved and they look absolutely nothing alike.
Sarah Polley and the CGI monster with the side set eyes that played her daughter in Splice.
Peggy Lipton and Mischa Barton as mother and daughter in Skipped Parts.
Ray Wise and Grace Zabriskie as Sheryl Lee's parents in Twin Peaks/Fire Walk With Me.
David Morse and Rory Culkin as father and son in Down In The Valley.
Lily Tomlin and Lili Taylor as mother and daughter in Short Cuts.
Julianne Moore and Eddie Redmaybe as mother and son in Savage Grace.

When they cast the BRADY BUNCH, the child actors were cast first. Sherwood Schwartz had four sets of child actors cast. One set was blond boys, another set brunet boys, one set blonde girls and the last set brunette girls. Schwartz wanted a brunet father/blonde mother or a blond father/brunette mother combination. One he cast Robert Reed and Joyce Bullifant (who was replaced later by Florence Henderson), Schwartz knew which child actors he was going to use.
Another story; Mary Tyler Moore auditioned for the role of Danny Thomas' daughter on MAKE ROOM FOR DADDY. He told her that with her nose, no one would believe that she was his daughter. He did remember her when it came time to cast the Dick Van Dyke Show.